This invention relates to a machine part of excellent super long life fatigue characteristics and, more in particular, it relates to a machine part used suitably for those portions undergoing repetitive high stresses (such as bending, tension and compression), for example, axles of vehicles such as bullet trains, gears, blades of turbines, reduction gears such as of automobiles and industrial machines, and bearings.
Heretofore, when fatigue fractures by repetitive stresses (such as the bending stresses, tensile stresses and compressive stresses) do not occur over the number of repetitive cycles N of 107 on metal materials such as high strength steels, it is considered that the fatigue fractures do not occur thereafter. Therefore, fatigue limits (the fatigue strength) are determined at the number of repetitive cycles: N=107 cycles.
In recent years, however, a phenomenon that fatigue fractures do not occur up to 107 cycles but fatigue fractures occur in excess of number of repetitive cycles N of 107 (the phenomenon is referred hereinafter as super long life fatigue fracture and the fatigue characteristic the number of at repetitive cycles : N=107 over is referred to as super long life fatigue characteristic) was reported by Naito, et al (Materials, 32, 361 (1983)) and Emura, et al (KIRON A-55, 509 (1989)), which has now attracted attention.
Since machine parts, for example, axles of vehicles such as bullet trains and blades of turbine are sometimes used under repetitive stresses in excess of 107 cycles, it is important to analyze the cause for the occurrence of the super long life fatigue fracture.
On the other hand, adverse effects of hydrogen on the static strength of high strength steels are well known as the phenomena such as delayed fracture. However, the effects of hydrogen have been pointed out as factors to reduce the fatigue characteristics of high strength steels, for the first time, by Murakami, et al only recently (The Society of Materials Science, Japan 24th Fatigue Symposium Proceedings (1998), and Materials, 48.10 (1999)).
As the method for preventing the deterioration of delayed fracture characteristic there are disclosed, for example, a method of trapping hydrogen intruding in steels thereby restricting the number of sulfide compounds and inclusions that form concentration sources of hydrogen (Japanese Published Unexamined Patent Application No. 1746/1998) or a method of dispersing and precipitating fine carbides, nitrides, sulfides in steels to trap intruded hydrogen in a dispersed manner thereby suppressing hydrogen embrittlement (Japanese Published Unexamined Patent Application No. 110247/1998). Such examples concern mainly for steel materials used in applications where a relatively great amount of hydrogen may possibly intrude from the surface of the steel materials (diffusive hydrogen) during use.
Also, Japanese Published Unexamined Patent Application No. 256274/1999 discloses high strength fine steel wires intended for the improvement of delayed fracture characteristic by restricting the amount of hydrogen intruding in the steel such that the amount of hydrogen released upon heating from a room temperature to 300xc2x0 C. is 0.5 ppm or less.
However, none of the methods disclosed in Japanese Published Unexamined Patent Application No. 1746/1998 and Japanese Published Unexamined Patent Application No. 256274/1999 is intended for the improvement of the super long life fatigue characteristic but for the improvement of usual fatigue characteristic (delayed fracture). Further, they are not based on the data that distinctly analyze the mechanism of hydrogen on the fatigue characteristic. Accordingly, even when the method described above is used, it is difficult to sufficiently improve the super long life fatigue characteristic.
In view of the above, this invention intends to solve the problem in the prior art as described above and has a subject of providing a machine part of excellent super long life fatigue characteristic.
In order to solve the subject described above, this invention comprises the following constitution. That is, the machine part according to this invention is characterized in that it is constituted with steel having a carbon content of 0.2% or more and the hydrogen content after hardening by heat treatment is 0.04 ppm or less.
Further, it is preferred the surface of the machine part is made to hardness of Hv 450 or more by a method, for example, of applying surface hardening treatment.
In accordance with the constitution, the machine part described above has extremely high reliability since there is less possibility of causing undetermined deterioration of fatigue strength due to the effect of hydrogen and the super long life fatigue characteristics are excellent.
Accordingly, the machine part according to this invention can be applied suitably to various machine parts which are used under repetitive stresses of over 107 cycles by rotations or vibrations, such as bearing rings or rolling elements of rolling bearings.
The present inventors have accomplished this invention based on the following findings obtained as a result of extensive studies for the effects of inclusions and hydrogen on the super long life fatigue fracture of high strength steels.
Present inventors have reported that non-metallic inclusions are present at the fracture starting points of steel test specimens suffering from super long life fatigue fracture, and regions appearing more dark under metal microscopic observation because of the rough surface state are present at the periphery of the inclusion (hereinafter referred to as ODA: Optically Dark Area) and hydrogen has an important role to the formation thereof (KIRON A-66, 642 (2000)).
FIG. 1 shows a scanning electron microscopic (SEM) photograph observing inclusions at the fracture starting point, ODA and the vicinity thereof in a test specimen (made of SCM 435) suffering from super long life fatigue fracture. As shown in FIG. 1, while typical fatigue fracture surface is served in the martensitic tissue at the outside of the ODA, the martensitic tissue is not distinctly observed in the ODA and a tissue that appears more fragile than usual fatigue fracture surface is observed.
ODAs around the inclusion (refer to FIG. 1) are observed in a test specimen fractured at long life by a low stress fracture test but is not observed in a test specimen fractured at a short life by a high stress fracture test. It is supposed from the foregoings that the ODAs were caused as a result of discontinuous development of cracks by the mechanism similar with delayed fracture by hydrogen trapped to the periphery of the intrusion and the repetitive stresses.
Then, the following test was conducted in order to investigate a relation between the dimension of the ODA and the hydrogen content. That is, a heat treatment (quenching) for the test specimen was conducted in a hydrogen-containing atmosphere (for example, in Rx gas) or in vacuum, and a fatigue test was conducted for each of the test specimens. When the fracture starting points of the test specimens fractured at about an identical fracture life were observed, the dimension of the ODA present at the periphery of the inclusion as the fracture starting point was considerably smaller in the specimen applied with quenching after heating in vacuum than in the test specimen applied with quenching after heating in the hydrogen-containing atmosphere. From the result described above, it has been found that a correlation exists between the dimension of the ODA and the hydrogen content.
The present inventors have already proposed an estimated equation for estimating the fatigue limit depending on the dimension of the non-metallic inclusion in order to evaluate the effect of the dimension of the non-metallic inclusion (defect) on the fatigue strength (Metal fatigue: Micro-defect and Inclusion, 1993, Yokendo), and it is possible to forecast the fatigue strength of a member according to the estimated equation. The estimated equation uses the dimension of the non-metallic inclusion, that is, the square root for the area of the non-metallic inclusion (hereinafter referred to as {square root over ( )}area) as a parameter.
The present inventors have found that the fatigue fracture occurs when the dimension of the non-metallic inclusion ({square root over ( )}area) and the dimension of the sum for the non-metallic inclusion and the ODA (square root of the sum for the area of the non-metallic inclusion and the area of the ODA; hereinafter referred to as {square root over ( )}areaxe2x80x2) exceed the limit value defined by the estimated equation.
In other words, they have found that a longer life can be expected when the dimension for the sum of the non-metallic inclusion and the ODA ({square root over ( )}areaxe2x80x2) is made smaller by the reduction of hydrogen content. As the measure for indicating the dimension of the non-metallic inclusion and ODA, ({square root over ( )}areaxe2x80x2)/({square root over ( )}area) is used preferably.
In addition, the present inventors have found that there is a tendency that the dimension of the ODA is larger (that is, the value: ({square root over ( )}areaxe2x80x2)/({square root over ( )}area) is larger) as the fatigue life is longer in a case where the hydrogen content exceeds 0.04 ppm. That is, the fatigue fracture occurs when the value of ({square root over ( )}areaxe2x80x2)/({square root over ( )}area) is larger. From the foregoings, for longer life, it is desirable that the value of ({square root over ( )}areaxe2x80x2)/({square root over ( )}area) at the repetitive number of cycles: N=5.0xc3x97107 is 1.5 or less.
Further, the restriction on the hydrogen content described above is particularly effective in a case where the carbon content in the steel constituting the machine part undergoing the repetitive stresses is 0.2% or more and the hardness is Hv 450 or more.
That is, the dimension of the ODA can be decreased when the carbon content is 0.2% or more, the hardness is Hv 450 or more and, further, the hydrogen content is 0.04 ppm or less in the steel constituting the machine parts undergoing the repetitive stresses.
Accordingly, they undergo fatigue below the limit value for the estimated value of the fatigue limit calculated by the estimated equation in view of the dimension of the defect (non-metallic inclusion) and longer life can be attained. Also, when the dimension of the non-metallic inclusion is restricted in the step of designing the machine part, the fatigue limit of the machine part can be estimated at long life and a machine part of high reliability excellent in the super long life fatigue characteristic can be expected.
Then, it is possible to forecast, in the stage of design, those adaptable to various machine parts such as gears, bearings, turbines and axles to be used frequently while undergoing repetitive stresses over 107 cycles by rotations and vibrations.
The machine parts in accordance with the invention means those parts constituting equipments such as prime movers and operation machines that convert the energy supplied from outside to specified useful jobs. For example, they mean axles of vehicles such as bullet trains, gears, turbines, and pumps. In addition, they can be applied to outer rings, inner rings, and cages constituting rolling bearings, as well as rolling elements constituting rolling devices such as linear guide bearings and ball screws.